i think the moment when it was released "Custom Product Builder", i was one of the first who tried it.
15+ years later...............Not even looked into the code or remember anything.
The contribution NOT will go follow your expectations.
The dev started with a guy who wanted to sell "desktop computers".
Back in that time, it was much easier to tell what was possible and what was not ( in relation to a desktop pc)
The add-on never evolved to be used with "any" kind of product.
Yet............ it is (still) a very interesting concept.
The economy just made it cheaper to buy pre-fabricated items in the meantime.
You will go work on an unfinished add-on.
Lots of changes required.